The SIG Model 1920 submachine gun was a licensed copy of the modified Bergmann / Schmeisser MP.18/I submachine gun. It was produced in Switzerland by famous arms-making factory SIG in Neuhausen, between 1920 and 1927. SIG Model 1920 differed from its German-made prototype by having magazine of greater capacity, and different calibers (7.65 Parabellum and 7.63 Mauser). Each SIG model 1920 submachine gun was marked "Brevet Bergmann" (Bergmann patent) on the top of the magazine housing. The version of the SIG M1920 in 7.65 Luger caliber was sold to Finland. Versions in the 7.63 Mauser caliber were sold to Japan and China. In 1930 SIG introduced an improved version of this design, which differed in the placement of the magazine (on the right side), and also had a detachable forward vertical grip below the stock. This was made to avoid users gripping the long magazine with left hand, which could result in feeding problems.

The SIG Model 1920 "Brevet Bergmann" submachine gun is a blowback operated, full automatic only weapon that fires from open bolt. Tubular receiver is attached to the front of the wooden stock, and could be pivoted barrel down for maintenance and disassembly. Magazine is inserted from the left (right on Model 1930) side. The manual safety consisted of the L-shaped cut, made at the rear of the cocking handle slot. This cut was used to lock the bolt in its rearward position by putting the cocking handle into the cut. Barrel was enclosed into the tubular jacket, perforated for better cooling. SIG submachine guns, purchased by Japan, were also fitted with bayonet mount. The rifle-type stock was made from wood. The rear sight was of tangent type, adjustable from 100 to 1000 meters range.